The existing tests for transform_view::iterator weren't quite right,
and can be simplified now that we have more of C++20 available to us.
Having done that, let's use the same pattern for iota_view::iterator
The `transform_view` test had been commented out because we were missing `std::is_eq` (D110515). However, it turned out that the commented-out test was a bit bogus for a couple other reasons:
- it lacked coverage for the rewritten operators and `operator==`
- it was a bit overcomplicated (especially now that `std::views::transform` has landed)
- it wrongly assumed that the `ContiguousView` from "types.h" had non-three-way-comparable iterators
Note the design decision here to rely on the fact that (for now) `random_access_iterator` has no `operator<=>`. If we ever add one, this test's sanity-check will fail, and we'll have to go find a different legacy iterator to use instead.